This invention relates to a subscriber line circuit which comprises a battery feed circuit and is for connection to a subscriber substation, such as a telephone set, through a two-wire line and to an input and an output terminal of an exchange.
Such a subscriber line circuit, also called a subscriber line interface circuit, is for use in an exchange office in supplying a battery feed current to the subscriber substation from a DC power source which is usually a storage battery installed in the exchange office. The subscriber line circuit is furthermore for use in transmitting a first communication signal from the subscriber substation to the input terminal of the exchange and a second communication signal from the output terminal of the exchange to the subscriber substation. The first and/or the second communication signal is usually superimposed on the battery feed current. Inasmuch as such a superposition is delivered between the subscriber line circuit and the subscriber substation through the two-wire line, the battery feed current and/or the first and/or the second communication signal will herein be named a loop signal. The exchange may either be a telephone exchange or a data exchange.
As will later be described with reference to one of twelve figures of the accompanying drawing, a conventional subscriber line circuit of the type described comprises a hybrid transformer comprising, in turn, first through third windings. The first winding is for connection to the subscriber substation and is used as a part of the battery feed circuit which additionally comprises resistors for limiting the battery feed current. First ends of the second and the third windings are for connection to the input and the output terminals of the exchange, respectively. Second ends of the second and the third windings are connected to each other and to a balancing network. A combination of the hybrid transformer and the balancing network is primarily for carrying out two-wire/four-wire conversion.
The hybrid transformer must comprise a large-sized and accordingly heavy magnetic core. The resistors are appreciably heated by the battery feed current and thereby result in a considerably poor utilization factor of the DC power source particularly when the subscriber substation is not far away from the exchange office. The hybrid transformer and the resistors have made it difficult to manufacture the subscriber line circuit by resorting to the integrated semiconductor circuit technique.